Multi-channel optical signals typically comprise a plurality of spectral channels, each having a distinct center wavelength and an associated bandwidth. The center wavelengths of adjacent channels are spaced at a predetermined wavelength or frequency interval, and the plurality of spectral channels may be wavelength division multiplexed to form a composite multi-channel signal of the optical network. Each spectral channel is capable of carrying separate and independent information. At various locations, or nodes, in the optical network, one or more spectral channels may be dropped from or added to the composite multi-channel optical signal, as by using, for example, a reconfigurable optical add-drop multiplexer (ROADM).
Reconfigurable optical add-drop architectures utilize a wavelength-separating-routing (WSR) apparatus and methods employing an array of fiber collimators providing an input (output) port and a plurality of output (input) ports; a wavelength-separator; a beam-focuser; and an array of channel micromirrors. Reconfigurable optical add-drop architectures are disclosed in commonly assigned U.S. Pat. Nos. 6,549,699, 6,625,346, 6,661,948, 6,687,431, and 6,760,511, the disclosures of which are incorporated by reference herein.
In operation, a multi-wavelength optical signal emerges from the input port. The wavelength-separator separates the multi-wavelength optical signal into multiple spectral channels; each characterized by a distinct center wavelength and associated bandwidth. The beam-focuser focuses the spectral channels into corresponding spectral spots. The channel micromirrors may be microelectromechanical system (MEMS) mirrors that are positioned such that each channel micromirror receives one of the spectral channels. MEMS generally refers to any of a number of mico-scale electromechanical devices that are typically fabricated using material deposition and etching techniques similar to those used in semiconductor integrated circuit manufacture. The channel micromirrors are individually controllable and movable, e.g., continuously pivotable (or rotatable), so as to reflect the spectral channels into selected output ports. As such, each channel micromirror is assigned to a specific spectral channel, hence the name “channel micromirror”. And each output port may receive any number of the reflected spectral channels. A distinct feature of the channel micromirrors in this architecture, in contrast to those used previously, is that the motion, e.g., pivoting (or rotation), of each channel micromirror is under analog control such that its pivoting angle can be continuously adjusted. This enables each channel micromirror to scan its corresponding spectral channel across all possible output ports and thereby direct the spectral channel to any desired output port.
The above-mentioned U.S. patents also refer to a means for achieving optimal coupling by using a dither scheme. Dithering a MEMS mirror is a means of determining the peak coupling of an optical communication path. The amount of dither used is a tradeoff between adding an unwanted disturbance to the optical path and having sufficient dither signal for servo control. Unfortunately, the process involved in building a MEMS device results in parameter variations from mirror to mirror. It would be advantageous to tune each MEMS mirror to avoid the process variation.
The Port and Channel servos on certain WSS systems, such as the WP4500 from Capella Photonics, Inc., of San Jose, Calif., use a dither tone to determine the direction and amount the servos will move the micromirror. The dither tone frequency is typically a sinusoid. The dither tone and a copy of the dither tone that is 90-degrees out of phase with the dither tone are used to dither a micromirror about two orthogonal axes. The resulting optical signal detected by the OCM is demodulated into sine and cosine components that are synchronously demodulated to form the control signals for the MEMS driver.
A disadvantage of the simple sinusoidal dither tone is that exogenous signals such as mechanical vibration or network optical modulation may be interpreted by the control system as a valid dither tone. In this case the mirrors can miss-position in response to the exogenous tone. It is the object of this invention to describe a method which is not susceptible to exogenous or alien, tones.
It is within this context that embodiments of the present invention arise.